Tagatose is a carbohydrate obtainable by isomerization of galactose. However, its existing content in nature is very low, and its characteristics are seldom known other than a few properties such as degree of sweetness, quality of taste and energy amount. The degree of sweetness of tagatose is about 90% of sugar, and its quality of taste is similar to fructose, and tagatose provides very fresh and sharp sweetness. Further, its energy amount is assumed to be 1.5 kcal/g, and it can be used as a low calorie sweetener.
As a method for producing tagatose crystals, a method wherein an organic solvent such as ethanol is added to a high purity tagatose solution for crystallization has been known. For example, Japanese International Publication No. 5-504256 discloses a method for obtaining tagatose crystals by adding seeds of tagatose crystals to a tagatose-containing syrup which contains ethanol.
However, since detailed characteristics of tagatose e.g. solubility of tagatose in water have not been known, there has been no report about a method for crystallization of tagatose from an aqueous system using no organic solvent.